The detection and avoidance of lead-contaminated herbage by dairy cows.

Lead from industry, and vehicles in some countries, can contaminate cattle pastures. We determined the concentrations of lead on herbage that cows can detect. Four cows were trained in a four-lane maze to detect an herbage sample that was superficially contaminated with 240 mg Pb/kg from three uncontaminated samples, and four were trained to detect an uncontaminated sample from three contaminated samples. Pb concentrations were reduced until the cows could not discriminate between samples, which occurred at 30 to 60 mg/kg. In a second experiment, 12 cows were allocated to two groups that grazed either pasture with added Pb (86 mg/kg) or pasture with no added Pb for 4 wk. Lead reduced grazing time and pasture biting rates, leading to an increase in herbage height. Immediately afterwards, these cows were grazed for 4 wk on a mosaic of 18 plots with no added lead, a low Pb level (67 mg/kg), or a high Pb level (102 mg/kg) (Experiment 3). Lead reduced grazing time and pasture biting rate, particularly at the high level. Plots without added Pb were grazed to the shortest height, and plots treated with Pb at 102 mg/kg were tallest. Prior exposure to Pb increased grazing time, especially on the high Pb level, and it increased the rate of biting pasture without Pb. To understand these phenomena, herbage samples from Experiments 2 and 3 were incubated in rumen liquor and artificial saliva, and gas production was measured over 48 h. In both experiments, the application of Pb reduced the rate of gas production or total gas production. It is concluded that cows can detect Pb on herbage, that it reduces their grazing time and pasture biting rate, and that they prefer to graze pasture without Pb. The aversion to Pb on pasture, which may be due to adverse effects on ruminal digestion, diminishes over time.